608 research outputs found
A seismologically consistent compositional model of Earth's core
International audienceEarth's core is less dense than iron, and therefore it must contain " light elements, " such as S, Si, O, or C. We use ab initio molecular dynamics to calculate the density and bulk sound velocity in liquid metal alloys at the pressure and temperature conditions of Earth's outer core. We compare the velocity and density for any composition in the (Fe–Ni, C, O, Si, S) system to radial seismological models and find a range of compositional models that fit the seismo-logical data. We find no oxygen-free composition that fits the seismological data, and therefore our results indicate that oxygen is always required in the outer core. An oxygen-rich core is a strong indication of high-pressure and high-temperature conditions of core differentiation in a deep magma ocean with an FeO concentration (oxygen fugacity) higher than that of the present-day mantle. mineral physics | first principles | geophysic
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Charge exchange in collisions of beryllium with its ion
Close-coupling calculations of the resonance and near resonance charge exchange in ion–atom collisions of Be+2 at low and intermediate energies are presented. Accurate ab initio calculations are carried out of the Born–Oppenheimer potentials and the non-adiabatic couplings that are due to the finite nuclear masses and drive the near resonance charge exchange. We show that the near resonance charge exchange cross section follows Wigner's threshold law of inelastic processes for energies below 10−8 eV and that the zero temperature rate constant for it is 4.5 × 10−10 cm3 s−1. At collision energies much larger than the isotope shift of the ionization potentials of the atoms, we show that the near resonance charge exchange process is equivalent to the resonance charge exchange with cross sections having a logarithmic dependence. We also investigate the perturbation to the charge exchange process due to the non-adiabatic interaction to an electronic excited state. We show that the influence is negligible at low temperatures and still small at intermediate energies despite the presence of resonances.Astronom
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Collisions of Trapped Molecules With Slow Beams
We present a theoretical study of molecular-trap loss induced by collisions with slow atomic beams based on an explicit analysis of collision kinematics in the laboratory frame and a rigorous quantum description of atom-molecule scattering in external fields. The theory is applied to elucidate the effects of nonuniform magnetic and optical trapping fields on low-temperature collisions of OH molecules with He atoms. Our calculations quantify the extent to which both elastic and inelastic cross sections are suppressed by external trapping fields, clarify the role of small-angle scattering in trap loss, and may benefit future experiments on collisional cooling of molecules in electromagnetic traps. The calculated cross sections for trap loss in He + OH collisions are consistent with recent experimental observations at low beam energies [ B. C. Sawyer Phys. Rev. Lett. 101 203203 (2008)], demonstrating the importance of including the effects of nonuniform trapping fields in theoretical simulations of cold collision experiments with trapped molecules and slow atomic beams.Astronom
Influence of population size, density, and proximity to talent clubs on the likelihood of becoming elite youth athlete
New perspectives on the evolution of within-individual genome variation and germline/soma distinction
Genomes can vary significantly even within the same individual. The underlying mechanisms are manifold, ranging from somatic mutation and recombination, development-associated ploidy changes and genetic bottlenecks, over to programmed DNA elimination during germline/soma differentiation. In this perspective piece, we briefly review recent developments in the study of within-individual genome variation in eukaryotes and prokaryotes. We highlight a Society for Molecular Biology and Evolution 2020 virtual symposium entitled "Within-individual genome variation and germline/soma distinction" and the present Special Section of the same name in Genome Biology and Evolution, together fostering cross-taxon synergies in the field to identify and tackle key open questions in the understanding of within-individual genome variation
Feedback from central black holes in elliptical galaxies. III: models with both radiative and mechanical feedback
We find, from high-resolution hydro simulations, that winds from AGN
effectively heat the inner parts (~100 pc) of elliptical galaxies, reducing
infall to the central SMBH; and radiative (photoionization and X-ray) heating
reduces cooling flows at the kpc scale. Including both types of feedback with
(peak) efficiencies of 3 10^{-4} < epsilon_mech < 10^{-3} and of epsilon_rad
~10^{-1.3} respectively, produces systems having duty-cycles, central SMBH
masses, X-ray luminosities, optical light profiles, and E+A spectra in accord
with the broad suite of modern observations of massive elliptical systems. Our
main conclusion is that mechanical feedback (including all three of energy,
momentum and mass) is necessary but the efficiency, based on several
independent arguments must be a factor of 10 lower than is commonly assumed.
Bursts are frequent at z>1 and decline in frequency towards the present epoch
as energy and metal rich gas are expelled from the galaxies into the
surrounding medium. For a representative galaxy of final stellar mass ~3
10^{11} Msun, roughly 3 10^{10} Msun of recycled gas has been added to the ISM
since z~2 and, of that, roughly 63% has been expelled from the galaxy, 19% has
been converted into new metal rich stars in the central few hundred parsecs,
and 2% has been added to the central SMBH, with the remaining 16% in the form
hot X-ray emitting ISM. The bursts occupy a total time of ~170 Myr, which is
roughly 1.4% of the available time. Of this time, the central SMBH would be
seen as an UV or optical source for ~45% and ~71$% of the time, respectively.
Restricting to the last 8.5 Gyr, the burst occupy ~44 Myr, corresponding to a
fiducial duty-cycle of ~5 10^{-3}.Comment: 41 pages, 11 figures (bitmapped, low-quality), ApJ accepte
AMUSE-Virgo II. Down-sizing in black hole accretion
(Abridged) We complete the census of nuclear X-ray activity in 100 early type
Virgo galaxies observed by the Chandra X-ray Telescope as part of the
AMUSE-Virgo survey, down to a (3sigma) limiting luminosity of 3.7E+38 erg/s
over 0.5-7 keV. The stellar mass distribution of the targeted sample, which is
mostly composed of formally `inactive' galaxies, peaks below 1E+10 M_Sun, a
regime where the very existence of nuclear super-massive black holes (SMBHs) is
debated. Out of 100 objects, 32 show a nuclear X-ray source, including 6 hybrid
nuclei which also host a massive nuclear cluster as visible from archival HST
images. After carefully accounting for contamination from nuclear low-mass
X-ray binaries based on the shape and normalization of their X-ray luminosity
function, we conclude that between 24-34% of the galaxies in our sample host a
X-ray active SMBH (at the 95% C.L.). This sets a firm lower limit to the black
hole occupation fraction in nearby bulges within a cluster environment. At face
value, the active fraction -down to our luminosity limit- is found to increase
with host stellar mass. However, taking into account selection effects, we find
that the average Eddington-scaled X-ray luminosity scales with black hole mass
as M_BH^(-0.62^{+0.13}_{-0.12}), with an intrinsic scatter of
0.46^({+0.08}_{-0.06}) dex. This finding can be interpreted as observational
evidence for `down-sizing' of black hole accretion in local early types, that
is, low mass black holes shine relatively closer to their Eddington limit than
higher mass objects. As a consequence, the fraction of active galaxies, defined
as those above a fixed X-ray Eddington ratio, decreases with increasing black
hole mass.Comment: Accepted for publication in ApJ (no changes wrt v1
On the Hydrodynamic Interplay Between a Young Nuclear Starburst and a Central Super Massive Black Hole
We present 1D numerical simulations, which consider the effects of radiative
cooling and gravity on the hydrodynamics of the matter reinserted by stellar
winds and supernovae within young nuclear starbursts with a central
supermassive black hole (SMBH). The simulations confirm our previous
semi-analytic results for low energetic starbursts, evolving in a
quasi-adiabatic regime, and extend them to more powerful starbursts evolving in
the catastrophic cooling regime. The simulations show a bimodal hydrodynamic
solution in all cases. They present a quasi-stationary accretion flow onto the
black hole, defined by the matter reinserted by massive stars within the
stagnation volume and a stationary starburst wind, driven by the high thermal
pressure acquired in the region between the stagnation and the starburst radii.
In the catastrophic cooling regime, the stagnation radius rapidly approaches
the surface of the starburst region, as one considers more massive starbursts.
This leads to larger accretion rates onto the SMBH and concurrently to powerful
winds able to inhibit interstellar matter from approaching the nuclear
starburst.
Our self-consistent model thus establishes a direct physical link between the
SMBH accretion rate and the nuclear star formation activity of the host galaxy
and provides a good upper limit to the accretion rate onto the central black
hole.Comment: 20 pages, 6 figures, accepted for publication in The Astrophysical
Journal
HST NICMOS Photometry of the reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan 4
We present results from NICMOS Hubble Space Telescope observations of the
reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan
4, obtained through the filters F110W and F160W (nearly equivalent to J and H).
For the first time the turnoff region of Liller 1 and the main sequence of
Terzan 5 and Terzan 4 are reached, as well as the horizontal branch of UKS 1.
The magnitude difference between the turnoff and the red horizontal branch
is used as an age indicator. From
comparisons with new isochrones in the NICMOS photometric system, we conclude
that the two metal-rich clusters NGC 6528 and Terzan 5 are coeval within
uncertainties () with 47 Tucanae. Liller 1 and UKS 1 are confirmed as
metal-rich globular clusters. Terzan 4 is confirmed as an interesting case of a
metal-poor cluster in the bulge with a blue horizontal branch.Comment: 7 pages, 6 figures, accepted for publication in A&
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